1. Field of the Invention
This invention relates to a packing structure for sealing a joint in a pipeline for conveying various kinds of refrigerant gas such as freon or carbon dioxide, or a volatile liquid such as gasoline, or hydrogen or oxygen for a fuel cell. More particularly, it relates to a packing structure to get the compatibility between a reliable seal even at a low temperature and excellent barrier properties (impermeability) to a refrigerant gas and a volatile liquid.
2. Description of the Related Art
An annular packing member such as an O-ring, or a plane packing member is usually used for sealing a joint in a pipeline for conveying various kinds of fluids. The packing has usually been a molded and cured product of a material having resistance to any such fluid. For example, the packing for sealing a fuel pipeline, such as for gasoline, has been properly and selectively made of fluororubber (FKM), fluorosilicone rubber (FVMQ), nitrile rubber (NBR), hydrogenated nitrile rubber (H-NBR), a mixture of acrylonitrile-butadiene rubber and polyvinyl chloride (NBR-PVC) or a mixture of FKM and FVMQ, and the packing for sealing an oil pipeline, such as for lubricant oil, has been properly and selectively made of acrylic rubber (ACM) or silicone rubber (Q) in addition to any such rubber.
If the pipeline is for a fluid used in a motor vehicle, the packing is required to maintain its sealing property even at a considerably low temperature, since the vehicle is likely to go to a very cold place. It has, however, been found that the ones which have excellent barrier properties among the known packing materials harden at a low temperature and fail to form a reliable seal at a low temperature because of an unevenness on metal or resin joint surface.
A metal joint 1 usually has an uneven surface with very small concavities 2 even if it may have been accurately finished, as shown in FIG. 1A. A resin joint also has a similar uneven surface due to the shrinkage which may occur during its injection molding. The concavities 2 form paths for fluid leakage between the joint and a packing member 3, such as an O-ring, though they are usually closed when the packing member 3 is deformed in the directions of an arrow y by a fluid pressure bearing upon it in the directions of an arrow x, as shown in FIG. 1B. The packing member 3, however, fails to form a reliable seal at a low temperature not reaching the glass transition point of its material, since it becomes too hard to be satisfactorily deformed to close the paths completely, as is obvious from FIG. 1C. These drawbacks are particularly remarkable when packing materials are FKM, H-NBR or the like having excellent barrier properties.
In the case that the fluid is a refrigerant gas or a volatile liquid such as gasoline, the packing is required to be highly impermeable to a refrigerant gas and a volatile liquid from the point of view of requirement of supplying no refrigerant gas into a cooler system and of preventing any freon gas, or gasoline from flowing out into the environment. It has, however, been found that the known packing materials with a good seal even at a low temperature, are not satisfactory in fluid impermeability. For example, the use of packing materials having rubber elasticity even at a low temperature (e.g. FVMQ or NBR) prevents loss of a seal at a low temperature, but fails to provide good fluid impermeability.
Thus, there is not known any packing material that can form a good seal even at a low temperature and is highly impermeable to any refrigerant gas, or volatile liquid.
It is, therefore, an object of this invention to provide a packing structure which can form a reliable seal even at a low temperature and is highly impermeable to any such fluid.
According to a first aspect of this invention, packing structure comprises a core element which is highly impermeable to a refrigerant gas, or volatile liquid, and a thin coated rubber layer formed on the core element and having a glass transition temperature lower than the lowest temperature at which the packing structure can be used (up to xe2x88x9235xc2x0 C.).
The core element is satisfactorily impermeable to any such fluid for preventing it from flowing out through the packing structure, and the rubber layer formed thereon ensures the maintenance of a reliable seal even at a low temperature, since it retains rubber elasticity even at the lowest temperature that is allowable for the structure. Even if the impermeability of the rubber layer to a volatile liquid is insufficient, it hardly affects the fluid impermeability of the packing structure as a whole, since the core element having excellent barrier properties occupies the greater part of the overall thickness of the structure. Moreover, the rubber layer is deformable to fit even a rough metal or resin joint surface closely to form a tight seal.
According to a second aspect of this invention, the packing structure is used for sealing a pipeline in a motor vehicle, and its rubber layer has a loss modulus (Exe2x80x3) showing a peak at a temperature not exceeding xe2x88x9235xc2x0 C., and a storage modulus (Exe2x80x2) not exceeding 2xc3x971010 dyn/cm2 at xe2x88x9235xc2x0 C. The rubber layer has a sufficiently low glass transition temperature and a sufficiently high flexibility at a low temperature to make the packing structure suitable for use with a fuel pipeline in a motor vehicle.
According to a third aspect of this invention, the rubber layer has a thickness of 4 to 50 xcexcm. Its thickness as stated is sufficiently large to close any concavities existing on a surface of a joint metal, and is sufficiently small not to affect the fluid impermeability of the packing structure as a whole. The inventors of this invention have found that such concavities usually have a depth of, say, 0.1 to 3 xcexcm and not exceeding 10 xcexcm in a mechanically finished metal joint surface, or a depth not exceeding, say, 12 xcexcm in an injection molded resin joint surface having shrinkage, or a surface made uneven by the separation of a coating layer. The fluid impermeability of the packing structure as a whole depends on the material and thickness (or diameter) of its core element and the material and thickness of its rubber layer. The inventors have found that the packing structure has a satisfactorily high fluid impermeability as a whole if its rubber layer has a thickness occupying, say, 0.2 to 2.5% of the thickness of its core element. If the core element has a diameter of 2 mm which is common to any packing member used in a fuel pipeline, it follows that it is desirable for the rubber layer to have a thickness of 4 to 50 xcexcm. This range of layer thickness has been found to be sufficiently large to fill the depth of concavities and unevenness as mentioned above.
According to a fourth asepct of this invention, the core element is of a cured product of fluororubber (FKM), hydrogenated nitrile rubber (H-NBR) or a mixture of acrylonitrile-butadiene rubber and polyvinyl chloride (NBR-PVC), or a fluoro, polyamide or polyester resin having a melting point at a temperature not lower than 140xc2x0 C., and the rubber layer is of epichlorohydrin rubber (ECO), nitrile rubber (NBR), low nitrile H-NBR, chloroprene rubber (CR), urethane rubber (U), fluorosilicone rubber (FVMQ), chlorosulfonated polyethylene rubber (CSM), chlorinated polyethylene rubber (CPE), chlorinated butyl rubber (Cl-IIR), brominated butyl rubber (Br-IIR), acrylic rubber (ACM), a mixture (NE) of NBR and an ethylene-propylene-diene terpolymer (EPDM), or ethylene-propylene rubber (EPR). The core element as set forth is of rubber having a particularly high level of impermeability to a fuel, or like fluid, or a resin having a melting point at a temperature not lower than 140xc2x0 C. The resin is comparable or even superior to fluororubber in fluid impermeability, is high in toughness, and is even more flexible at a low temperature than fluororubber having a higher temperature dependence of flexibility. The rubber layer as set forth is of rubber having a particularly good sealing property at a low temperature and adhering closely to the core element so that no separation thereof resulting in a broken seal may occur.
Therefore, the effect of the first or second aspect of the invention is especially clear in the fourth aspect of the invention to prevent effectively the loss of the seal by the separation between the core element and the coating rubber.
According to a fifth aspect of this invention, the core element and the rubber layer are directly cured under heat to adhere to each other. They adhere to each other without the aid of any adhesive. Their adhesion is stronger than when an adhesive is used, and their separation is less likely to occur. No peeling or shaving of the rubber layer is caused by the joint with which the packing structure is used.
According to a sixth aspect of this invention, the rubber layer has a surface resistivity not exceeding 106 xcexa9/sq. There has been recently an increasing demand for e.g. a fuel pipeline which is not electrically charged. However, it has been difficult to add a satisfactorily large amount of an electrically conductive material to any known packing formed of a single material, such as fluororubber or a resin. The addition of an electrically conductive material may make fluororubber much less easy to work upon to form e.g. an O-ring and may yield a product which is too hard to make an effective seal and be satisfactory in mechanical properties. The rubber layer on the packing structure of this invention is, however, easy to make electrically conductive to the extent that it may have a surface resistivity not exceeding, say, 106 xcexa9/sq. That level of electrical conductivity is sufficient for preventing a pipeline from being electrically charged, and can be achieved without the sacrifice of the sealing or other properties of the packing structure, since the core element is principally responsible for its fluid impermeability, while the rubber layer serves as an electrical conductor.
The above and other advantages of this invention will become more apparent from the following description and the accompanying drawings.